iOS, formally known as iPhone OS, is the operating system for all iDevices.
iPhone OS began in and was announced alongside the original iPhone at the Macworld Conference & Expo on . Later that year, on , the original iPhone was released to the world with (what is now) iPhone OS 1.0.
Initially, iOS didn't even have a name; It was initially just said to run a trimmed down version of Mac OS X (now macOS). With the release of the iPhone OS 2 and iPhone SDK, Apple dubbed it iPhone OS. Initially chosen as it was what ran on the iPhone, it later was utilized by the iPod touch, iPad, and Apple TV (second generation and newer). As such, iPhone OS was renamed "iOS" with the fourth major version. This name was utilized for all iDevices for many years.
With the release of the original Apple Watch on , Apple released watchOS, a trimmed down version of iOS that's designed for the small screen of the Watch. Unlike the (then unreleased) tvOS and iPadOS which kept version number parity with iOS, the original Apple Watch was released with watchOS 1.0, based on iOS 8.2.
Later that year, with the release of the fourth generation Apple TV in , Apple released tvOS 9 alongside iOS 9. tvOS, like watchOS, is (internally) still iOS, but designed for a 10-foot user interface. The second and third generation Apple TVs however, continued to use the "iOS" branding.
Four years after the release of iOS 9, with iOS 13, Apple again rebranded iOS, but this time for their iPad line of products. The resulting operating system was named iPadOS. According to Apple, the change was made to differentiate the iPad from the iPhone and iPod touch lines as it (iPadOS) would contain a greater emphasis on multitasking.[AppleUnveilsIPadOS]
iOS (and watchOS, tvOS, and iPadOS) features a hybrid kernel known as XNU. It is mostly programmed in ARM assembly, C, C++, and Objective-C. As XNU is Unix-like, it uses "union mounting" (compared to the Windows way of using drive letters). The flash is partitioned into two separate blocks: one for the operating system (mounted at /) and one for the user data (mounted at /private/var).
Over the years, iOS has required more and more space as the operating system has become more complex. Initially, iPhone OS required less than half a gigabyte to function, but with almost each major release, the root filesystem partition grows bigger. Currently, iOS 13 will claim over six gigabytes for itself.
iOS features many built-in apps. Some have been available since the inception (such as Phone), others removed over time, and others added as time goes on.
/Applications folder. Apps not included are, but not limited to: GarageBand, iMove, iTunes U, Keynote, Numbers, and Pages.| App | Availability | Description |
|---|---|---|
| Apple Store | 13.0? - present | Apple Store |
| App Store | 2.0 - present | App Store client |
| Books (previously iBooks) | 8.0 - present | PDF and eBook viewer |
| Calculator | 1.0 - present | Scientific calculator |
| Calendar | 1.0 - present | Calendar |
| Camera | 1.0 - present | Camera |
| Clips | 13.0 - present | Short video (clip) editor |
| Clock | 1.0 - present | World clocks, alarms, and timers |
| Compass | 3.0 - present | Compass and (in later versions) spirit level |
| Contacts | 1.??? - present | Contact book |
| Demo App | 1.0 - present | Used in demo displays to display a video when the device is not in use |
| FaceTime | 7.0 - present | FaceTime telephony |
| Files | 13.0 - present | iCloud file manager |
| Find My (previously Find My iPhone and Find My Friends) | 9.0 - present | Device and friend locator |
| Game Center (app) | 4.1 - 9.3.6 | Game Center account manager |
| Health | 8.0 - present | Health informatics |
| Home | 8.0 - present | HomeKit based devices manager |
| iTunes Store (previously just iTunes) | 1.1 - present | iTunes Store client |
| 1.0 - present | Email manager | |
| Maps | 1.0 - present | Apple Maps (Google Maps on 5.1.1 and below) |
| Measure | 11.0 - present | AR based measuring app |
| Messages (previously Text and SMS) | 1.0 - present | Texting (SMS) and (since iOS 5) iMessage |
| News | 9.0 - present | News |
| Nike+iPod | 2.2.1 - 8.4.1 | Nike+ receiver |
| Notes | 1.0 - present | Note taking app |
| Phone | 1.0 - present | Phone |
| Photos | 1.0 - present | Photo library |
| Podcasts | 8.0 - present | Podcast library |
| Reminders | 6.0 - present | Reminders manager |
| Safari | 1.0 - present | Web browser |
| Settings | 1.0 - present | Device settings manager |
| Setup | 5.0 - present | Device setup; Runs after a restore or update |
| Siri | 4.0 - present | Siri |
| Stock | 1.0 - present | Stock price viewer |
| Tips | 8.0 - present | Device tips |
| TV | 13.0 - present | Apple TV+ client |
| Voice Memos | 3.0 - present | Audio recorder |
| Wallet (previously Passbook) | 6.0 - present | Apple Wallet |
| Watch | 8.0 - present | Apple Watch manager |
| Weather | 1.0 - present | Weather viewer |
| YouTube | 1.0 - 5.1.1 | YouTube |
Apple has maintained a steady release schedule with iOS' (and related) major releases; A new major version is released with every new flagship iPhone. Currently, iOS is on it's 13th major release with iOS 14 expected to be announced at WWDC 2020.
Each link below will take you to a page dedicated to that major version and its new features. In addition, each page will contain a full version history of that major version.